The invention disclosed and claimed herein generally pertains to an improved or enhanced distributed Multiple Access Control (MAC) for a radio network operating in an uncoordinated environment. More particularly, the invention pertains to a method and apparatus for a network control of the above type, wherein one of the network units or nodes is assigned certain supervisory functions. Even more particularly, the invention pertains to a network of the above type wherein resynchronization is made more robust, and network status and reconfiguration are communicated to all network units with significantly greater efficiency.
As is well known in the art, there is increasing interest in providing computers, telephones and other small electronic devices with the capability to connect and communicate wirelessly with one another, over short ranges, by means of radio links. Such capability could conceivably eliminate or substantially reduce the need for cables or infrared connections between devices such as computers, peripherals, telephones and headsets. Moreover, a number of devices could thereby be readily joined together to form small networks, or multiple networks, within a limited space. The assignee herein, a major supplier of mobile telecommunications equipment and systems, has initiated a program to develop a wireless communication capability of this type. This program, known as the “Bluetooth Short Range Radio System,” is now supported by a number of large electronics industry vendors and suppliers. Bluetooth is intended to operate in an uncoordinated environment, and is thus designed to communicate at 2.45 GHertz over the Industrial, Scientific and Medical (ISM) band, which is unlicensed and globally available.
The Bluetooth communication system provides ad-hoc connectivity between devices and is characterized by absence of a central coordinating unit. In Bluetooth, all devices are equal and any two units can set up a connection. In one implementation, Bluetooth uses a “master-slave” principle, wherein access to the communication channel is regulated by the master device in real-time. Although any device can take on the role of master, the master plays a central role, and in this implementation absence of the master results in temporary disintegration of the network.
Recently, work has been initiated to expand the Bluetooth system by providing it with a high-rate mode. This effort is referred to, for example, in U.S. patent application Ser. No. 09/385,024, filed Aug. 30, 1999. Moreover, in order to increase flexibility of traffic exchange on the high-rate channel, an alternative MAC scheme has been proposed. In the alternative arrangement, instead of a centralized control as in the master-slave configuration referred to above, a more distributed control has been proposed. This distributed control uses a protocol referred to as a Ping-Pong protocol between a multiplicity of devices, and is based on an implicit token exchange concept. This arrangement is referred to in further detail in U.S. patent application Ser. No. 09/710,204, filed Nov. 9, 2000.
The token based Ping-Pong protocol is illustrated in FIG. 1, which shows units A, B and C of a Bluetooth network at different times during a succession of time slots. In accordance with the Ping-Pong protocol, packets are transmitted between units, wherein each packet has a header indicating packet length or duration. Thus, FIG. 1 shows unit A commencing or initiating a transmission, which is directed to unit C. Unit C is allowed to transmit a packet after unit A has finished transmission. Unit C will know when it can commence transmitting, by monitoring the length information in the header of the packet sent by unit A. At the same time, unit B senses that the transmission from A is directed toward C. This enables unit B to stop receiving, until the end of the transmission from unit A. After the transmission from A to C, unit C sends a packet to unit B. Unit B is then allowed to send, after completely receiving the packet sent from C and the unit C transmission ends. This transmission concept can be considered to use a token mechanism, wherein the token is carried from one unit to another unit by the transmitted packet. The token thus resides at a unit for only a short period of time, until the next transmission carries it to the next unit.
In the token passing arrangement described above, transmission failures may occur from time to time. Accordingly, to ensure unconditional access to the channel and to restart the Ping-Pong mechanism after a transmission failure, priority slots are defined within the framework of the Ping-Pong protocol. The priority slots serve as time markers reserved for transmissions by an associated unit, and only the associated unit is allowed to transmit on the slot. Thus, the token automatically resides at the unit associated with the priority slot. This provides a way of giving units unconditional access to the communication channel. If the timing is such that two or more priority slots of different units coincide, the unit with the highest priority receives the right to send. FIG. 2 shows an example of this priority slot concept in combination with the Ping-Pong protocol, wherein unit C has priority slots PS at fixed intervals of eight slots. Thus, at each priority slot PS unit C is exclusively granted the right to transmit. This right is granted unconditionally, i.e., the token is given to unit C based on its ownership of the priority slot at a fixed point in time, not because unit C received the token in a previous packet reception.
From the above, it will be seen that the token can be obtained by either (1) being received by a unit along with a received packet, or (2) being granted to the unit corresponding to a priority slot. The Ping-Pong protocol and the priority slots collectively constitute a MAC mechanism for the radio channel of the above network. Synchronization in the network, based on this protocol, is maintained by tracking the header of each transmission.
Notwithstanding its benefits, the distributed MAC described above is characterized by some important limitations and disadvantages. For example, the network could have three units A, B and C, wherein unit A is able to receive transmissions from unit B but not from unit C, due to limited range or fading conditions. Thus, whenever unit C initiates a transmission upon receiving the token, unit A may lose synchronization. In order to resynchronize, unit A would have to make use of unit B priority slots or its own priority slots, possibly resulting in long periods of synchronization loss for unit A.
A further problem can arise if unit A communicates only with unit B and not with any other unit of the network. The Ping-Pong protocol allows this by letting unit A listen to the priority slots of unit B. In this situation, unit A would not be aware that a unit D was added to the network. This could become very important, if D had priority traffic to send and therefore had priority slots. Unit A would need to be aware of this network addition or reconfiguration, in order to avoid overriding the priority slots of unit D. Yet another problem could arise if unit A and unit B were to negotiate certain new parameters like creating new priority slots, allocating new addresses, or other network-related parameters, which could conflict with similar decisions made by units C and unit D simultaneously. This could result in a conflict in the allocation of priority slots. Moreover, unit C could receive the token but have no traffic to send. It could send the token to unit B or unit D, but could not send it to unit A due to limited range. This could result in an unfair situation where unit A was “starved,” or deprived of opportunity to transmit.